The present invention relates to a part-time four-wheel drive vehicle which is arranged to improve fuel economy.
A four-wheel drive vehicle is superior in its ability for hill climbing and ability for rough road driving. If, however, all four wheels are always driven at the same speed, a four-wheel drive vehicle cannot be turned smoothly. During a turn at a large steering angle of the steerable wheels, the front wheel of an inner or outer side must travel along a circle having a large radius and the rear wheel of the same side must travel along a small circle. Because of these differences in turning radius, there arises a large difference between a rotational speed (an average rotational speed, to be exact) of the front wheels and a rotational speed (an average rotational speed) of the rear wheels. If a vehicle in a four-wheel drive mode is forced to turn through a tight corner, abnormally great torques are exerted on the ends of front and rear propeller shafts and axle shafts. Therefore, the steering of the vehicle becomes very heavy, and there arises a danger of damage to the propeller shafts and axle shafts. Furthermore, the front wheels and the rear wheels slip in the opposite directions during a turn, so that the tendency to understeer is increased, and there arises an abnormal tire friction (called tight corner brake) which tends to brake the vehicle and causes an engine stall.
To overcome these handling and tire friction problems, some four-wheel drive vehicles are provided with a changeover means capable of changing the state of the four-wheel drive system between a two-wheel drive mode and a four-wheel drive mode. This type of four-wheel drive is called a part-time type. A part-time type four-wheel drive vehicle can be turned through a tight corner smoothly by holding the drive system in the two-wheel drive mode.
Some four-wheel drive vehicles of the part-time type are further provided with locking clutches. Each of the locking clutches is provided between an axle shaft and a part-time drive wheel which is disconnected from the engine when in the two wheel drive mode. Each of the locking clutches is automatically engaged to connect the part-time drive wheel and its axle shaft when the vehicle is driven in the four-wheel drive mode, and cannot be disengaged until the axle shaft is rotated in the opposite direction. Thus, the locking clutches of this type can avoid a detrimental and dangerous shock which would be exerted on the drive system if the locking clutches are engaged or disengaged while the vehicle is moving. Examples of such locking clutches are disclosed in Japanese Patent provisional publications No. 53-147152 and No. 54-108315, and Nissan service shuho published by Nissan Motor Company Limited, No. 461(160-2), pages 37-40.
However, a part-time four-wheel drive vehicle having such locking clutches has some disadvantages. The locking clutches of this type are held engaged unless the axle shafts are rotated in the reverse direction by moving the vehicle backwards. Therefore, there is a large possibility that the locking clutches remain engaged while the vehicle is moving in the two-wheel drive mode, and accordingly the axle shafts, the propeller shaft and other members of the drive system are driven uselessly by the part-time drive wheels which are disconnected from the engine and roll on the road surface, resulting in deterioration of fuel economy of the vehicle and occurrence of disagreeable noises. In order to know whether the locking clutches are engaged or disengaged, it is necessary to shift the drive system to the four-wheel drive mode while the vehicle is moving. The driver can estimate that the locking clutches are engaged if the drive system is smoothly shifted to the four-wheel drive mode during vehicle movement. If gear noises are produced by this shifting operation, the driver can estimate that the locking clutches are disengaged. Such a shifting operation is very troublesome for the driver.